1. Field of the Invention
Embodiments of the invention relate generally to the field of networking. More particularly, an embodiment of the invention relates to methods of and apparatus for fiber-to-the-home (FTTH) RF over Glass (RFoG) Architecture and customer-premise-equipment (CPE).
2. Discussion of the Related Art
Telephone companies such as Verizon and AT&T have started to offer services over fiber-to-the-premise (FTTP) and fiber-to-the-curb (FTTC) systems such as FiOS™ and U-Verse™. These systems offer dramatically higher data bandwidths by bringing optical fiber to the home or close to home. In order to maintain their upper hand in bandwidth per customer, North American cable operators started deploying scalable fiber-to-the-home (FTTH) systems, building upon fiber deployed to date in new builds and upgrades that can offer similar to, or higher than, bandwidths provided by FiOS™ and U-Verse™.
MSOs want to continue utilizing DOCSIS platform for wideband services such as high speed data, Voice over IP (VoIP) and other services supported by this platform, which provides for downstream data bandwidth up to 640 Mb/s or more, until such a time as yet higher data speeds are required. At such a time, the MSOs want the flexibility to upgrade their FTTH CPE device to handle Gb/s data speeds offered by passive optical networks (PONs) such as GPON or GEPON. They also want to support deployed interactive TV services that are based on set top boxes with active upstream signaling to support fully interactive services such as Video on Demand (VoD) and Switched Digital Video (SDV).
RF over Glass (RFoG) is the name given to the generic FTTH architecture that supports both legacy DOCSIS cable upstream signals and an optional future expansion to additional high speed (>1 Gb/s) PON service. However, deploying cost-effective RFoG system makes future expansion of this system with GPON or GEPON more difficult. The RFoG transmitters used to transmit upstream DOCSIS signals and set top box upstream signaling information for interactive TV, and placed in the CPE utilize a low-cost 1310 nm laser, which is the same wavelength as that used by upstream PON signals. The solution has been to use a different wavelength, usually 1590 nm, to transport the cable upstream signal and 1310 nm to transport the upstream PON signal. For systems that initially deployed 1310 nm upstream lasers, the expansion would result in replacing and obsolescing these deployed lasers with much higher cost CPE devices.
FIG. 1 shows the schematic diagram of the customer-premise-equipment (CPE) device typically used by cable operators to provide both traditional cable service and PON service on an RFoG system expanded to support PON architecture and services. The CPE uses one optical filter to extract the traditional cable services (1550 nm downstream/1590 nm upstream) and a second optical filter to extract the PON service (1490 nm downstream/1310 nm upstream).
The CPE device typically uses a relatively low-cost digital-quality 1310 nm laser for transmitting the upstream baseband PON signal from the optical network unit (ONU) but a significantly more expensive 1590 nm laser (or other CWDM wavelength such as 1610 nm) for transmitting the traditional cable return signals. The optical receivers for the downstream signals (1550 nm for the cable downstream and 1490 nm for the PON downstream) are relatively low-cost in comparison to the upstream lasers.
A disadvantage of this conventional RFoG architecture is the disproportionate cost of transporting the traditional cable return signals—mainly signaling from a set-top-box (STB) and QAM channels for DOCSIS data signals. This is due to the fact that the 1310 nm is the standardized wavelength used to carry the upstream PON data. Therefore, another wavelength such as 1590 nm (or a nearby CWDM wavelength such as 1610 nm) is typically used. However, such lasers are currently significantly higher in price than 1310 nm lasers due to much more stringent requirements on standard CWDM lasers in comparison to generic 1310 nm lasers. Low cost generic lasers with wavelengths that would not collide with the remaining three wavelengths are not currently available.
The cable return signals lie in a narrow frequency band (for example 5-42 MHz in North America and 5-65 MHz in Europe, generally F9-F8) and have a typical maximum upstream data capacity of 120 Mb/s to 240 Mb/s, about five to ten times less than the capacity of a PON service. However, the cost of the 1590 laser used to transport the cable return signals is almost twice that of the 1310 nm laser used to transport the upstream PON signal. Thus the cost per unit bandwidth for transporting the cable upstream signals is about ten to twenty times higher than for the upstream PON signal.
This seems too much of a premium to pay for a relatively low-bandwidth signal. However, cable operators have expressed a strong preference for keeping their existing STB and DOCSIS infrastructure and for adding future high-speed Ethernet data service using overlay architecture such as RFoG.